The present invention relates generally to direction finding using a continuously variable time delay line, and more generally to the field of fiber optics.
In addition to their use as the transmission medium in telecommunications systems, optical fibers are useful for radar signal processing, particularly where large time-bandwidth products and/or electromagnetic interference immunity are required. By employing taps along the length of a fiber in various configurations, several signal processing devices such as band pass filters, discriminators and code generators/matched filters can be implemented.
One technique for passive RF direction finding is based on the interferometric detection of RF phase delays using an antenna array. This technique, however, is highly frequency dependent. This problem can be overcome by using the tapped fiber optics matched delay filter which operates in the time delay mode rather than the phase delay mode. This approach yields RF arrival directions which are independent of frequency. Since it operates in the time delay mode, the fiber optic filter for direction finding is frequency independent over a very large range.
The use of optical fibers as delay lines is well documented. See, for example, S. A. Pappert, M. N. McLandrich, and C. Chang, Journal of Lightwave Technology, Vol. LT-3, p. 273, Apr. 1985. The delay incurred by an optical pulse propagating through a fiber of given length is based on the transit time. By launching a pulse of light into a fiber, the transit time of the pulse through the fiber results in a delay equal to the transit time for transmission. Another method creates twice the transit time and relies on reflection. Both methods have been used to create time delays for various applications. Since the length of the fiber is fixed, the temporal delay is determined by the speed of light in vacuum and the group index of refraction.
The group index N.sub.g can be approximated by the following equation ##EQU1## where n.lambda. is the refractive index of the fiber at the wavelength .lambda. and dn.sub.80 /d.lambda. is the dispersion of the fiber waveguide material contained in the fiber core. Dispersion is the change in the refractive index with wavelength. It can be readily seen from the above equation that variations in the group index are a result of dispersion of the fiber waveguide material. Thus a variable time delay can be obtained with a fixed length of optical fiber by changing the wavelength of the light pulse propagating through the fiber.
A patent by Soref, U.S. Pat. No. 4,671,604, describes a wavelength dependent tunable optical time delay technique using simple dispersion. By varying the wavelength of the pulse, a variable time delay can be obtained with a fixed length of optical fiber. The Soref patent proposes using high dispersion waveguide material in the optical fiber. A single, discretely tunable optical source is used to produce wavelength tuning. This technique does not allow continuous wavelength tuning, and therefore only stepwise variations in the optical delay are achieved.
A patent by Goutzoulis, U.S. Pat. No. 5,101,455, describes a tunable fiber optic delay line in which an array of discrete optical sources is used, each producing a different wavelength. The requirement of numerous individual optical sources precludes continuous tunability and therefore precludes continuous variation of the delay time. Additional complications of the technique described by Goutzoulis involves the complexity of fabricating the array of fibers and the difficulty of producing a laser array with the proper wavelength emissions.
A fiber optic variable delay line was described by Soref in U.S. Pat. No. 4,671,605. A plurality of optical fibers of varying lengths was used to produce a variable delay. The time of travel of the optical signal is determined by which fiber the optical signal passes through. The practicality of such a technique is limited by the requirement that many fibers have to be cut to precise lengths and the reliance upon numerous bidirectional coupling elements.
A patent by Schuss et al., U.S. Pat. No. 4,164,373, describes a fiber optic spectrometer that is used to spectrally analyze the wavelength content of a pulsed light source. The broadband light from the source is dispersed prior to entering a plurality of optical fibers. The spectrometer is designed so that each optical fiber transmits a different wavelength band emitted by the source. All fibers are the same physical length. Owing to dispersion, the optical packets transmitted by each fiber arrive at the detector at different times. This technique therefore provides a wavelength-to-time conversion technique which can be used to determine the spectral content of the pulsed source.
A patent by Kapron et al., U.S. Pat. No. 3,988,614, describes a technique for compensating the dispersion introduced by transmission of a broad spectral pulse in an optical fiber. In this technique the pulse spreading caused by dispersion is corrected by applying different time delays to the optical signals representing different wavelengths. If a broad spectral pulse of short duration is propagated through an optical fiber, dispersion introduces wavelength dependent delays for each spectral component of the pulse. The result of this is that a temporally broadened pulse emerges from the fiber. In the Kapron patent, the broadened pulse is dispersed, and each wavelength band is directed to a separate optical fiber. The length of each fiber is adjusted to introduce a compensating delay for a given band of wavelengths. In this manner the difference in pulse delay experienced by each wavelength band in the initial fiber transmission can be canceled.
Chromatic dispersion is a result of the variation of the group velocity of light with wavelength. The dispersion in a fiber optic is due to the dispersion of the material composing the waveguide. For light in the 850 nm to 900 nm range, the differential delay over a 10 nm spectral bandwidth is about 1 ns per km of fiber. This delay is calculated for fused silica, a commonly used waveguide material for optical fibers. Other waveguide materials may have higher dispersion and differential delays of 2 ns per km or more can be obtained. Using the relatively broad spectral emission of a light emitting diode (LED) as an example, a spreading of 2 to 10 ns per km can occur in a fused silica optical fiber.
In an optical fiber light travels approximately 20 cm/ns. An incremental delay of 1 ns would therefore require 20 cm of additional fiber. Alternatively, for a 1 km long fused silica fiber, an incremental delay of 1 ns requires that the spectral source tune over a range of 10 nm. Higher dispersion optical waveguide material might require only 5 nm of tuning for a 1 ns incremental delay.
Laser diodes, which are a convenient and widely used transmitters for fiber optics, cannot tune over a wavelength range greater than a few nm unless the junction temperature is varied. Unfortunately, changing the junction temperature is a slow process. Therefore, for rapid wavelength tuning, current control is a more appropriate technique.
Cleaved coupled cavity laser diodes, which are substantially more complicated and expensive than "ordinary" laser diodes, can produce tuning ranges of 30 nm or more. For a fused silica fiber this would provide approximately 3 ns of incremental delay, while for more dispersive waveguide materials as much as 6 ns incremental delay might be achieved. In either event, using the techniques described above to achieve incremental time delays exceeding several ns would require a large number of fibers of varying lengths. In addition, a laser diode or other optical source that is capable of a wide tuning range would be required.
Therefore, a need exists in the state of the art for producing a directional finding apparatus using a continuously variable time delay line which can produce large incremental time delays with small changes in wavelength and is convenient to use with an ordinary laser diode optical source.